A univariate halting test using Fisher's exact test.
f_halt(condition, covariates, thresh)
The ratio of the p-value and the threshold, or 0 if the p-value is less than the threshold.
A factor vector containing condition labels.
A columnwise matrix containing covariates to match the conditions on.
The return value of halting_test has to be greater than or equal to thresh for the matched groups.